Class XIV myosins play an important role in the life cycle of apicomplexan parasites of medical and/or veterinary importance, such as Plasmodium spp, Toxoplasma gondii, Sarcocystis neurona and Cryptosporidium parvum. One example of an apicomplexan parasite is Toxoplasma gondii, which is a member of the phylum Apicomplexa and is a common infectious agent of humans that can result in health risks to immune-compromised individuals and the developing fetus. This obligate intracellular parasite must penetrate a host cell and replicate to survive. The invasive stage of the parasite relies on a unique form of substrate-based motility called gliding motility, which is driven by a class XIVa myosin motor, TgMyoA that is powered by an actomyosin-based complex, which is called the “glideosome”. The class XIV myosin TgMyoA heavy chain is one of eleven myosin heavy chains found in T. gondii (1) and is an essential component of the glideosome, which is necessary for efficient parasite motility, invasion, and egress from the host. Parasites lacking TgMyoA are avirulent in a mouse model of parasite infection (2).
The TgMyoA motor is located between the plasma membrane and the inner membrane complex (IMC), a double membrane that is continuous around most of the cell (3). TgGAP50 (a 50 kDa gliding associated protein), an integral membrane glycoprotein of the IMC, acts as a membrane receptor for the motor (4); TgMyoA is linked indirectly to TgGAP50 through an apicomplexan-specific N-terminal extension of its regulatory light chain, TgMLC1, and TgGAP45 (a 45 kDa gliding associated protein). Several other proteins, including TgGAP40 (a 40 kDa gliding associated protein), TgGAP70 (a 70 kDa gliding associated protein), and TgELC1 (a putative essential light chain) have recently been identified as additional components of this myosin motor complex (5, 6). The mechanism by which the motor complex generates motility has remained unclear.